1. Field of the Invention
This invention relates to an optical amplifier which amplifies a signal light propagating in an optical waveguide by means of laser glass doped with a rare earth.
2. Description of the Related Art
A prior art optical amplifier employing a glass fiber doped with a rare earths is constituted as typically shown in FIG. 1. In the figure, the numeral 40 denotes an amplifier substrate; the numerals 41 and 42 denote an optical fiber for signal light transmission; the numeral 43 denotes an optical fiber doped with a rare earth element; the numeral 44 denotes a light source typically formed with a laser, for emitting a pumping light; the numeral 45 denotes a multiplexer; and the numeral 46 denotes a demultiplexer. A signal light of 1.536 .mu.m wavelength incident to optical fiber 41 is input to optical fiber 43 doped with Er (erbium). A pumping light of, for example, 1.49 .mu.m or 0.98 .mu.m wavelength is introduced by multiplexer 45 so as to transmit into optical fiber 43 together with the signal light. During the transmission in optical fiber 43 doped with rare earth, electrons in the rare earth element are pumped up by the pumping light so as to amplify the signal light due to a stimulated emission effect. The amplified light is transmitted via demultiplexer 46 into optical amplifier 42. Pumping light which is unnecessary now is removed by demultiplexer 46 so as to be abandoned. Direction of the pumping light propagation in the rare-earth-doped optical fiber 43 may be chosen opposite, i.e. backward, to the signal light propagation. Moreover, demultiplexer 46 may be replaced with an isolator of the pumping light.
The above-described prior art optical amplifier employing the optical fiber doped with the rare earth requires relatively large number of components and requires as long as several meters or more than one hundred meters of the rare-earth doped optical fiber 43. Moreover, the multiplexer requires its precise alignment; therefore, it has been difficult to satisfy the requirements for compactness as well as high reliability for the use in, for example, a submarine cable system.